1. Field of the Invention
The present invention relates to a mirror for use in a lithographic apparatus, a lithographic apparatus, a device manufacturing method, and a device manufacture thereby.
2. Description of the Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that circumstance, a patterning device, such as a mask, may be used to generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. including part of one or several dies) on a substrate (e.g. a silicon wafer) that has a layer of radiation-sensitive material (resist). In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion in one go, and scanners, in which each target portion is irradiated by scanning the pattern through the projection beam in a given direction (the “scanning” direction) while synchronously scanning the substrate parallel or anti-parallel to this direction.
Lithographic apparatus may soon employ extreme ultraviolet (EUV) sources, which may use tin or another metal vapor to produce EUV radiation. This tin may leak into the lithographic apparatus, and will be deposited on mirrors in the lithographic apparatus, e.g., the mirrors of the radiation collector. The mirrors of such a radiation collector may be multilayered and may have a ELV reflecting top layer of ruthenium (Ru). Deposited layers of more than approximately 10 nm tin (Sn) on the reflecting Ru layer will reflect EUV radiation in the same way as bulk Sn. A layer of 10 nm Sn may be deposited very quickly near a Sn-based EUV source. The overall transmission of the collector will decrease significantly, since the reflection coefficient of tin is much lower than the reflection coefficient of ruthenium.